LEDs are current-controlled devices in the sense that the intensity of the light emitted from an LED is related to the amount of current driven through the LED. FIG. 1 shows a typical relationship of relative luminosity to forward current in an LED. The longevity or useful life of LEDs is specified in terms of acceptable long-term light output degradation. Light output degradation of LEDs is primarily a function of current density over the elapsed on-time period. LEDs driven at higher levels of forward current will degrade faster, and therefore have a shorter useful life, than the same LEDs driven at lower levels of forward current. It therefore is advantageous in LED lighting systems to carefully and reliably control the amount of current through the LEDs in order to achieve the desired illumination intensity while also maximizing the life of the LEDs.
LED illumination products have been developed which provide the ability to vary the forward current through the LEDs over an acceptable range in order to provide dimming capability. LED lighting systems have also been devised which, through the use of multiple colors of LEDs and individual intensity control of each color, can produce a variety of color hues. Systems incorporating Red, Green, and Blue LEDs can achieve near infinite color variations by varying the intensity of the Red, Green, and Blue color banks.
As LED Lighting Systems have become more prevalent, various methods have been devised to control the current driven through the LEDs to achieve dimming and color mixing. One common method is a Pulse Width Modulation (PWM) scheme such as that set forth in U.S. Pat. Nos. 6,618,031, 6,510,995, 6,150,774, 6,016,038, 5,008,595, and 4,870,325, all of which are incorporated herein by reference as if set forth in full. PWM schemes pulse the LEDs alternately to a full current “ON” state followed by a zero current “OFF” state. The ratio of the ON time to total cycle time, defined as the Duty Cycle, in a fixed cycle frequency determines the time-average luminous intensity. Varying the Duty Cycle from 0% to 100% correspondingly varies the intensity of the LED as perceived by the human eye from 0% to 100% as the human eye integrates the ON/OFF pulses into a time-average luminous intensity.
Although PWM schemes are common, there are several disadvantages to this method of LED intensity control. The fixed frequency nature of PWM means that all LEDs switch on (to maximum power draw) and off (zero power draw) at the same time. Large illumination systems can easily require several amperes of current to be instantaneously switched on and off. This can create two problems. First, the rapid on and off switching of the system can create asymmetric power supply loading. Second, the pulsing of the current through electrical leads can create difficult to manage electromagnetic interference (EMI) problems because such leads may act as transmitters of radiofrequency energy that may interfere with other devices operating at similar frequencies.
In order to address these problems with PWM, an alternate method of LED intensity control, called Frequency Modulation (FM) has been developed and implemented by Artistic Licence Ltd. and described at their website, particularly in Application Note 008, located at http://www.artisticlicence.com/(last visited Jun. 17, 2004).
The FM method of LED intensity control is similar to the PWM method in that the LEDs are switched alternately from a maximum current state to a zero current state at a rate fast enough for the human eye to see one integrated time-average intensity. The two methods differ in that PWM uses a fixed frequency and a variable pulse width (duty cycle), whereas FM delivers a fixed width pulse over a variable frequency. Both of these methods achieve a dimming effect through the varying ratio of LED ON time to OFF time. Where the FM method improves upon the PWM method, is in the fact that a varying frequency creates fewer EMI problems, and reduces the asymmetric power supply loading effect.
The FM method, however, suffers from the same drawbacks of the PWM method when the dimming level is held constant, or is changing at a relatively slow rate. In fact, at a constant level of dimming, it can be seen that the EMI and asymmetric power supply loading effects of PWM and FM are identical. As the size of the lighting system (total number of LEDs) controlled by a central control and power supply gets large, these negative effects can get correspondingly large and difficult to overcome.
There is a third prior art method of LED intensity control that eliminates the drawbacks of the PWM and FM techniques, called Analog Control. Analog Control is a method of varying the current being driven through the LEDs through a continuous analog range from zero through the maximum desired level. Since the LEDs are not constantly pulsed between two states of zero and maximum current, EMI problems are minimized, as are power supply loading problems associated with large instantaneous changes in power draw. An example of a prior art LED Analog Control circuit is shown in FIG. 2.
The Analog Control method, although solving the problems associated with PWM and FM techniques for LED driving, nevertheless has other drawbacks. Due to process variations and tolerances of analog components, including the LEDs themselves, variations in luminous intensity from the desired intensity, i.e., brightness control inaccuracies, can show up at lower levels of current where component tolerances make up a larger percentage of the total effect. In addition, wavelength shifts can occur especially at lower current levels, as shown in FIG. 3, which can lead to undesired color shifts in the light output by the LEDs. As lighting designers seek to employ very low levels of output illumination, a higher degree of control in this range becomes more and more desirable.
A circuit and control method has been devised for variably controlling the current through LEDs without the drawbacks inherent in PWM and FM schemes, and that overcomes the problems with the Analog Control circuit associated with low current levels that are described above. This method is set forth in U.S. Pat. No. 7,088,059 which is incorporated herein by reference as if set forth in full. The method described in U.S. Pat. No. 7,088,059 combines the analog and pulsed dimming schemes in one circuit, allowing for a combination dimming scheme which takes advantage of the positive aspects of each scheme, while minimizing the drawbacks of the individual methods. This prior art circuit is shown in FIG. 4, along with the charts in FIGS. 5, 6, and 7 which give an example of typical control values and the resulting relative illumination levels achieved with this circuit.
The method described in U.S. Pat. No. 7,088,059 is limited, however, in its implementation of the analog circuitry. It is adapted to a linear current limiting circuit in series with the LED load as can be seen in FIG. 4. In such a linear circuit, there can be significant power loss in terms of excess circuit heat generated in the linear device. The linear devices in the U.S. Pat. No. 7,088,059 circuit are MOSFETs (M10 and M20 in FIG. 4) operated in the active transistor region. The power dissipated in M10 and M20 increases nearly linearly with increases in the LED current (ID1 and ID2). Because of the lower power efficiency of such linear circuits, large power systems typically do not employ them. The latest LEDs operate at currents in the hundreds of milliamps and even above 1 amp, as compared to tens of milliamps for the LEDs employed in the circuit described in U.S. Pat. No. 7,088,059. As LEDs have increased in power and luminosity output, it has become common to employ driving circuits that are active, meaning the power delivered to the end system is dynamically adapted to the requirements of the load. This results in increased system efficiency and less heat dissipated by the driving circuitry. Such active driving circuits are commonly implemented using switching regulators configured as buck, boost, or buck-boost regulators with outputs that are set to constant-voltage, or constant-current depending on the circuit. Typically, in LED driving applications, the switching regulator circuit is adapted to sense the current through the LEDs, and dynamically adjust the output so as to achieve and maintain a constant current through the LEDs.
In prior art implementations, commercial products have been developed such as Boca Flasher's SBL and HPCCS products which use switching regulators as LED drivers wherein the switching regulator circuit is pulse-enabled with a digital signal (such as PWM) in order to achieve a dimming effect (in the case of a single channel) or color mixing (in the case of multiple channel configurations).
In light of the previously discussed disadvantages of PWM and other pulsed methods of LED intensity control, there is need in the art for a simple hybrid method of analog and pulse dimming such as that disclosed in U.S. Pat. No. 7,088,059, but which is adaptable to higher power systems such as those implemented with switching-regulator driver circuits. It is an object of the present invention to provide an efficient high power LED driver circuit utilizing common switching regulators, capable of dynamically varying the current delivered to the LEDs in proportional response to an analog voltage input. It is also an object of the invention to further vary the time-average value of the LED current in proportion to a digital pulsed input. It is a further object of the present invention to combine the analog and pulsed dimming control of the LED driver circuit in such a manner as to overcome the above discussed disadvantages of both analog current dimming, and pulsed dimming methods such as PWM and FM.